More than 100 Democratic lawmakers in Congress expressed support for a series of lawsuits that environmental and tribal groups filed last year over the Trump administration's decision to trim the size of two national monuments in southern Utah.

President Trump signed proclamations last December to reduce the size of the Bears Ears National Monument by 83% to 228,784 acres, down from 1.35 million, and for trimming the Grand Staircase-Escalante National Monument by 46% to about one million acres, down from 1.88 million. In response, several environmental and tribal groups filed five lawsuits in U.S. District Court for the District of Columbia to challenge the reductions.

In amicus briefs filed last Monday in support of the plaintiffs, 118 lawmakers -- 26 senators and 92 House members -- argued that Congress has been entrusted with protecting federal lands. They added that Congress, through the Antiquities Act of 1906, gave the president the authority to designate land as national monuments, but did not give the power to abolish or diminish the size of existing national monuments. That authority remains with Congress, they said.

"President Trump and his enablers don't seem to care what laws stand in the way of their anti-environmental agenda, and they need to be stopped here and now," said Rep. Raúl Grijalva (D-AZ). "These monument reductions are illegal and open the door to presidents ignoring Congress whenever they feel like it."

Grijalva, who is expected to become chairman of the House Natural Resources Committee in January, said "this case is a test of our basic separation of powers, and Republicans cheering on the president today may feel very differently if he wins the battle only to have a Democrat follow him in the White House."

Sen. Tom Udall (D-NM), ranking member of the Senate Appropriations Subcommittee on the Interior, Environment, and Related Agencies, said Trump had "overstepped the authority delegated to presidents by Congress, putting cherished national monuments like Bears Ears and Grand Staircase-Escalante on the chopping block in an egregious giveaway to big corporations."

The Trump administration had argued that previous administrations abused the Antiquities Act and sometimes made monument designations against the will of the local public.

Attorneys general for California, Hawaii, Maine, Maryland, Massachusetts, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington have filed separate amicus briefs in support of the plaintiffs.

Plaintiffs filed two cases, Wilderness Society et al. v. Donald J. Trump et al., No 17-2587, and Grand Staircase Escalante Partners et al. v. Donald J. Trump et al., No. 17-2591, to protest the reduction of the Grand Staircase-Escalante National Monument. The cases have since been combined. Meanwhile, three cases -- Hopi Tribe et al. v. Trump et al., No. 17-2590, Utah Diné Bikéyah v. Trump, No. 17-2605, and Natural Resources Defense Council v. Trump, No. 17-2606 -- were filed to oppose the reduction of the Bears Ears National Monument, and have also been combined.

Judge Tanya Chutkan is presiding over all five cases. Chutkan denied motions by the Trump administration to have the cases moved to Salt Lake City in September.